High-speed Living Anionic Polymerization of Methacrylic Esters with

Macromolecules 1993,26, 3403-3410

3403

High-speed Living Anionic Polymerization of Methacrylic Esters with Aluminum Porphyrin Initiators. Organoaluminum Compounds as Lewis Acid Accelerators Hiroshi Sugimoto, Masakatsu Kuroki, Tsuyoshi Watanabe, Chikara Kawamura,t Takuzo Aida, and Shohei Inoue* Department of Synthetic Chemistry, Faculty of Engineering, University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113, Japan Received January 11, 1993

ABSTRACT The polymerizationof methacrylicesters initiated with methylaluminumtetraphenylporphyrin (1, X = Me) proceeded dramatically rapidly at room temperature in the presence of sterically crowded organoaluminum compounds such as methylaluminum ortho-substituted diphenolatea (3a-3g), bis(triphenylmethanolate)(4b),and triphenylaluminum, where undesired reaction between the growing species (enolatoaluminum porphyrin, 2) and the Lewis acids was sterically suppressed. Use of lees hindered Lewis acids such as methylaluminum diphenolates having no ortho substituents (3h, 3i),bis(diphenylmethanolate) (4a), trimethylaluminum, and triisobutylaluminum for the polymerization of methyl methacrylate under similarConditionsresultedin terminationof chain growth. However,this unfavorableterminationwas avoidable when the Polymerization temperature was lowered to -40 O C .

Introduction Potential utility of Lewis acids for promoting nucleophilic reactions via coordinative activation of substrates is well established in the field of organic chemistry,' but undesired attack of nucleophiles to the Lewis acidic (electrophilic) center, leading to destruction of these species, is usually inevitable. Thus, the application of this concept to the control of anionic (nucleophilic) polymerization has been essentially limited. Hatada et al. have utilized a mixture of tert-butyllithium and trialkylaluminum for the polymerization of methyl methacrylate (MMA), where the chain growth takes place with Scheme I11 living character to give syndiotactic PMMA (poly(methy1 Accelerated Chain Growth methacrylate)) in a nonpolar solvent, but lowering the temperature to, e g . , -78 O C is a prerequisitee2 Very recently, another interesting example has been reported by Ballard et al., who have utilized tert-butyllithium as initiator in conjunction with a bulky Lewis acid such as 1 (X o-pdumor) 1a.u) diisobutylaluminum2,6-di-tert-butyl-4-methylphenolate, Nucleophile Lowi. Acid and achieved the living polymerization of methacrylic esters at 0 0C.3In these two examples, however, some aggregated species between the nucleophilic growing species and Lewis acids have been proposed as the active species. In a previous communication,4we have already reported that the polymerization of MMA initiated with methylaluminumtetraphenylporphyrin (1, X = Me, (TPP)AlMe), proceeding via an enolatoaluminum porphyrin as the growing species (2, Scheme I),sis dramatically accelerated by the addition of a bulky Lewis acid such as methyla(3b) (Scheme 11). luminum bis(2,4-di-tert-butylphenolate) On the basis of this finding, monodisperse,high molecular weight PMMA with M nexceeding one million was synthesized.6 The basic idea of this finding came from our interesting observation that the living anionic polymerization of 6-valerolactone (6-VL) via an alcoholatoaluminum porphyrin (1, X = 0-polymer) as the growing species is accelerated upon addition of a Lewis acidic chloroaluIn the present paper, organoaluminum-based Lewis minum porphyrin (1,X = Cl)7 which itself, however, has acids with different structures were employed for the no ability to initiate the polymerization (Scheme 111). polymerization of various methacrylic esters initiated with 1 (X= Me), and the mode of acceleration was discussed in terms of the type of Lewis acid and the polymerization t On leave from Kaneai Paint Co. LM., Higaahiyawata,Hiratsuka, conditions. Kanagawa 254, Japan. 0024-9297/93/2226-34Q3$Q4.OOIQ 0 1993 American Chemical Society

Macromolecules, Vol. 26, No. 13, 1993

3404 Sugimoto et al.

''I"

Scheme V

Scheme IV 2

t

Me3AI

--

*'

Men

' '

I

Me

I

2 MeH

i

-/AI\,&

I(

31

1 (X = Me)

Scheme VI

Experimental Section Materials. 5,10,15,20-Tetraphenylporphine(TPPHz) was synthesized from pyrrole (0.8 mol) and benzaldehyde (0.8 mol) in propionic acid (2.5 L) under reflux for 0.5 h, and the crude product, precipitated upon keeping the reaction mixture overnight at room temperature, was recrystallized from CHCMMeOH (1/2 v/v) to give TPPHz in 20% yield.8 Dichloromethane (CHzClz) was washed successively with concentrated H2S04,water, and aqueous NaHC09, dried over CaC12, and distilled over CaHz in a nitrogen atmosphere. Deuterated dichloromethane (CDaCld, stirred a t room temperature with a small amount of triethylaluminum, was subjected to several freeze-thaw cycles, and collected in a trap cooled with a liquid nitrogen bath and storedunder nitrogen. Benzene ( C A ) , deuterated benzene (Cas), hexane, and tetrahydrofuran (THF) were distilled in B nitrogen atmosphere over sodium benzophenone ketyl just before use. Diisopropylamine was distilled after refluxing over amixture of CaHz and KOH under nitrogen. tertButyl isobutyrate was fractionally distilled over CaHz under reduced pressure in a nitrogen atmosphere. 2-tert-Butylphenol, 4-tert-butylphenol, 2,4-di-tert-butylphenol,2,4,6-tri-tert-bu2-phenylphenol, and tylphenol,2,6-di-tert-butyl-4-methylphenol, 2,6-dichlorophenol were recrystallized from hexane. 4-Hydroxytriphenyhethanol, and anisole, 3-t ert-butyl-4-hydroxyanisole, diphenylmethanol were recrystallized from toluene. Methyl methacrylate (MMA), ethyl methacrylate (EMA), isopropyl methacrylate ('PMA), n-butyl methacrylate ("BMA), isobutyl methacrylate ('BMA),tert-butyl methacrylate (LBMA), benzyl methacrylate (BnMA), and dodecyl methacrylate (CW MA) were fractionallydistilled over CaHz under reduced pressure in a nitrogen atmosphere. Deuterated methyl methacrylate (MMA-ds) was prepared as follows: To a DzO solution (82.6 mL) of NaCN (0.96 mol) was added acetone-& (0.68 mol) at room temperature, followed by a mixture of D20 (88.3 mL) and D#04 (62.6 g, 0.63 mol) at 0 "C. The resulting mixture was stirred at 0 "C for 1h, and the organic layer extracted with ether was dried over magnesium sulfate, and subjected to fractional distillation under reduced pressure, affordingacetone cyanohydrin-d7in 63% yield (39.4 9). Acetone cyanohydrin-d7was added dropwise at 60 "C to 98% HzSO4 (34.1 g, 0.35 mol) containing fuming H&Od (15.1 g) in the presence of metallic Cu (1.5 g) and CuCl(0.5 g), and the mixture was stirred at 140 "C for 1h. After cooling to 100 "C, methanol-dr (0.78 mol) was added to the mixture, and stirring was continued for 20 h at the same temperature. Then, thereaction mixture was allowed to cool to room temperature, and water (49.6 mL) was added. The organic phase separated was dried over molecular sieves 4A, and fractionally distilled under nitrogen, affording MMA-dB in 10.4% yield (4.8 g), which was further distilled in the presence of triethylaluminum under nitrogen before use.9 Trimethylaluminum ( M e a ) and triisobutylaluminum ('BuBAl) were fractionally distilled under reduced pressure in a nitrogen atmosphere. Triphenylaluminum ( P w ) was synthesized as follows: To an ether solution (24 mL) of AlCh (53 mmol, 7.1 g) was added a hexane solution (1.8 M, 88 mL) of phenyllithium (159 mmol) at -78 "C by a hypodermic syringe in a nitrogen stream, and the mixture was stirred at -78 "C for 1 h under nitrogen and then allowed to warm to room temperature. After additional stirring for 30 min at room temperature, the reaction mixture was filtered under nitrogen to remove insolubleLiC1. Then, volatile fractions were removed under reduced pressure to leave a white powder, which was subjected to sublimation in vacuo, followed by recrystallization from benzene under nitrogen, affording P h d as white crystals (1.8 g, 13% yield based on AlC&).l0 Preparation of Methylaluminum 5,10,15,20-Tetraphenylporphine (1, X = Me) (Scheme IV). To a round-bottomed flask (50 mL) equipped with a three-way stopcock containing

BuO, Et\AI-CI Et'

+

Me

PuO,

Li-0

,Me

C=C,

7A1-d Et,

Me

LiCl

Et/

5

TPPHz (0.25 mmol) under dry nitrogen were successively added CH2C12 (10 mL) and M e a (1.2 equiv, 0.03 mL) by means of hypodermic syringes in a nitrogen stream, and the mixture was stirred for 1h in a nitrogen atmosphere. Then, volatile fractions were removed from the reaction mixture under reduced pressure to leave 1 (X = Me) as a purple powder." Preparation of Organoaluminum Olater. Methylalumi( 3 0 (Scheme V).To num Bis(2,4,6-tri-tert-butylphenolate) a 100-mL round-bottomed flask equipped with a three-way mmol, 10.7 stopcock, containing 2,4,6-tri-tert-butylphenol(40.8 g), were successively added hexane (52 mL) and M e d l (20.4 mmol, 1.96 mL) at 0 "C, and the mixture was stirred at room temperature for 1h, whereupon white precipitates were formed. The suspernatant was decanted, and the residual liquid phase (40 mL) separated was taken away by a syringe from the flask under nitrogen. Then, hexane (30 mL) was added to the residue at 60 "C, affording a clear solution, which was allowed to cool to room temperature, giving 3f as white crystals in 46% yield (5.2 g). The crystals were dried under reduced pressure at room temperature under nitrogen.l2 Other organoaluminum compounds (3a-3e,3g-3i, 4a, and 4b) were similarly synthesized by the reactions of M e d with the corresponding phenols or alcohols (2 equiv) in C& or CH&12 under nitrogen, and the reaction mixtures were used for polymerization. 1- tert-Butoxy- 1-[ (diethylaluminio)oxy]-2-methylpropene (5) (Scheme VI). To a 50-mL round-bottomed flask equipped with a three-way stopcock, containing a THF solution 0.7 mL), was added (4.5 mL) of diisopropylamine (5 "01, at -78 dropwise a hexane solution (3.1 mL) of "BuLi (5 "01) "C by a hypodermic syringe in a nitrogen stream, and the mixture was stirred at the same temperature for 30 min. Then, tertbutyl isobutyrate (5 mmoLO.82 mL) was added by means of a syringe to the reaction mixture at -78 "C. After stirring for 30 min at -40 "C, EtzAlCl (5 "01, 0.63 mL) waa added over a period of 5 min to the reaction mixture. Stirring the reaction mixture at -40 "C for 15 min led to the formation of 5 with deposition of LiCl.'* The resulting suspension was settled, and the supernatant liquid phase separated was used for polymerization. Polymerization. Polymerizationeof Alkyl Methacrylatee by the Methylaluminum Porphyrin4rganoaluminum Systems. A representative example is given below by the polymerization of methyl methacrylate ( M U ) with (TPP)AlMe (1, X = Me). To a 100-mL round-bottomed flask attached to a threeway stopcock containing a CH&ls solution (8mL) of 1 (X= Me, 0.2 "01) was added MMA (43.4 mmol,4.6 mL) by a syringe in a nitrogen stream. This mixture was illuminated by xenon arc light (300 W)through a filter to cut out light of wavelength shorter than 420 nm. After 2.5-h irradiation at 35 "C, methylaluminum bis(2,4-di-tert-butylphenolate)(3b) was added (3 equiv with respect to 1 (X= Me)) at room temperature under diffuse light. An aliquot of the reaction mixture was taken out by a syringe in a nitrogen stream, and subjected to 'HNMR and gel permeation chromatography (GPC) analyses to determine the monomer conversion and the average molecular weights of the produced polymer, respectively. Two-StagePolymerization of MMA in the Presence of Lewis Acid (30. To a 100-mL round-bottomed flask attached to a three-way stopcock containing a CHzCl2 solution (16 mL) of 1 (0.4 mmol) was added MMA (20 m o l , 2.1 mL) by a syringe in a nitrogen stream. The mixture was irradiated with xenon arc light (A > 420 nm) at 35 "C. After 3-h irradiation, methylalu-

Anionic Polymerization of Methacrylic Esters 3405

Macromolecules, Vol. 26, No. 13, 1993 Table I. Polymerization of Methyl Methacrylate (MMA) via an Enolatoaluminum Porphyrin (2, R = Me) in the Presence of Methylaluminum Diolates (MeAlXa). [MeAlXzld time/ con$/ run MeAlX2 [210 min % Muc M,,/Muc 1

2 3 4 5 6 7 8 9 10 11

3a 3b 3b 30 3d 3f 3g 3h 3i 4a 4b

3 0.2 2 3 3 3 0.5 0.5 0.5 3 3

0.5 3 10 0.5 0.75 5 0.5 60 60 60 60

100 100 100 100 92 100 74 7 8 6 91

21 500 26300 23700 22000 27 700 24300 31800

1.10 1.07 1.07 1.10 1.42 1.11 1.16

24000

1.04

In CHZC12 under nitrogen, [MMAIJil (X = Me)lo = 200,[l (X = Me)]o = 16.2 mM, [l (X= Me)lo = [2 (R = Me)lo. Determined by 1H NMR analysis of the reaction mixture. Estimated by GPC based on polystyrene standards. 0

minum bis(2,4,6-tri-tert-butylphenolate)(30 was added (1equiv with respect to 1) at room temperatureunder diffueelight. After 90a, an aliquot of the reaction mixture was taken out by a syringe in a nitrogen stream and subjected to NMR and GPC analyses to confiim the complete monomer consumption and to estimate the average molecular weights of the produced polymer, respectively. The polymerization mixture remaining in the flask was allowed to stand at 25 O C for 4 h, and then the second part of the MMA (80mmol) was fed. After 60 s, MeOH (5 mL) was added to the flask,and the reaction mixture was subjected to lH NMR and GPC analyses. Measuremente. Gel permeation chromatography (GPC) measurements were performed at 40 "C on a Toyo Soda Model 8020 high-speed liquid chromatograph equipped with a differential refractometer detector and a variable-wavelength UV-vis detector, using tetrahydrofuran as eluent with a flow rate of 1.0 mL min-1. The molecular weight calibration curve was obtained by using standard polystyrenes: Mn = 2 890 OOO (Mw/Mn = 1.09, 422 OOO (1.04),107 000 (1.07),43 900 (LOl), 16 700 (1.02),9000 (1.06),6200 (1.04), 4000 (l.lO),and2800(1.05). *Hand'3C NMR measurements were performed in C&e, CDC&,, or CDzClz using a JEOL type GSX-270 spectrometer, where the chemical shifts were determined with respect to C& (6 7.40), CHCl3 (6 7.28), or CHzClz (6 5.30),for 'H, and CDzClz (6 54.0)for lacas internal standards.

Results and Discussion Polymerization of Methacrylic Esters via Enolatoaluminum Porphyrins (2) in the Presence of Methylaluminum Diphenolates (3) and Dialcoholates (4). The polymerization of methyl methacrylate (MMA) was initiated with methylaluminum tetraphenylporphyrin (1,X = Me) under irradiation with visible light, and a t the monomer conversion of 6-8%, Lewis acids such as methylaluminum diphenolates (3a-3i) and methylaluminum dialcoholates (4a,4b)were added to thesystem (Table I). Among the methylaluminum diphenolates examined, 3a-3g having substituents at the ortho positions of the phenolate ligands were quite effective for acclerating the polymerization (runs 1-7), allowing the formation of narrow MWD PMMAs with Mn values in fair agreement with that (20 OOO) expected from the initial mole ratio of MMA to 1 (X= Me) of 200. In terms of both the power of acceleration and the cleanness of polymerization, the diphenolates having o-tert-butyl substituents (3a-3c, 3e, and 3 0 are the most favorable. On the other hand, when methylaluminum diphenolates having no ortho substituents (3h and 3i) were used, the polymerization was terminated just after they were added to the system (runs 8 and 9). As for methylaluminum dialcoholates (4a, 4b), the tertiary alcoholate (4b)gave a satisfactory result (run

Table 11. Polymerization of Methyl Methacrylate (MMA) via an Enolatoaluminum Porphyrin (2, R = Me) in the Premnce of Methylaldnum Bis(2,4-di-tert-butylp henolate) (3b).

[3b]d time con+/ magnitude of run 1210 /s % Mnc(Mu) Mw/Mnc accelerationd 5500 (4300) 1.09 1400 1 0.2 5 21 2 1.0 5 62 14400 (12 100) 1.10 14 000 46 200 3 3.0 3 100 25500(21700) 1.07

OIn CHzClz under nitrogen, [MMAldLl (X= Me)lo = 200, [I (X = Me)]o = 16.2 mM, [l (X= Me)lo = [2 (R = Me)lo. *Determined by 1H NMR analysis of the reaction mixture. Estimated by GPC based on polystyrene standards. Mole ratio of MMA reacted per second before and after addition of 3b. 7-

x

5

5.0

4.0

2.5

-

2.0

-

1.5

3

'

'

3.0

'

2.0

'

1.0

-

.

Figure 1. Polymerization of methyl methacrylate (MMA) initiated with (TPP)AlMe (1, X = Me) via an enolatoaluminum porphyrin (2, R = Me) in the presence of methylaluminum bis(2-tert-butyl-4-methoxyphenolate) (3c) ([3cld[210= 0.5)in CHr Clz at room temperature. Relationshipbetween Mn ( 0 )(Mw/Mn ( 0 ) )of the polymer and the initial monomer-to-initiator mole ratio ([MMA]J[l (X= Me)lo) at 100% conversion. 111,whereas use of the secondary alcoholate (4a)resulted in termination of chain growth (run 10). In runs 1-7, the dark red-purple color of the polymerization system, characteristic of the growing enolate species (2, R = Me), was observed to remain unchanged throughout the polymerization, while in runs 8-10, the polymerization systems immediately turned bright red-purple characteristic of 1 (X = OR, OAr) upon addition of the Lewis acids. In the polymerization of MMA initiated with 1 (X = Me) ([MMA]o/[llo = 200), the degree of acceleration became more pronounced as the amount of 3b was increased from 0.2 to 1.0 to 3.0 equiv with respect to 2 (R = Me) (Table 11). If the added 3b initiates the polymerization, the number of polymer molecules produced should increase proportionally to the amount of 3b, and consequently, the Mn value should decrease. However, irrespective of the ratio of 3b to 2, all the polymers formed were of narrow MWD,and the observed Mn values were always close to those expected from the mole ratios of the monomer reacted to 2. As shown in Figure 1,the M nof the polymer could be precisely controlled by changing the monomer-to-initiator ratio in the presence of 0.5 equiv of 3c with respect to the growing species 2 (R= Me). These results indicate that the added 3 does not initiate but only accelerates the polymerization. In this regard, all the produced polymers in Table I1 were silent in GPC when monitored at 263 nm (2,4-di-tert-butylphenol, ,A 263 nm; anisole, 269 nm), indicating no incorporation of the


3406 Sugimoto et al. Table 111. Polymerination of Methacrylic Esters via Enolatoaluminum Porphyrin9 (2) in the Presence of Methylaluminum Bis(2-tert-butyl-4-methoxyphenolate) (34' run monomer timels con@/% Mnc(M-) MwlMn' 1 EMA 30 100 27700(23000) 1.09 2 PMA 30 100 30900(25600) 1.10 3 "BMA 30 100 34900(28400) 1.07 4 'BMA 30 100 36300(28400) 1.07 5 'BMA 90 11 6 BnMA 90 100 32900(35200) 1.08 7 C12MA 90 64 30600(32600) 1.09 a In CH2Cl2 under nitrogen, [monomerldll (X= Me)ld[3cIo = 200/1.0/0.5, [l (X= Me)Jo = 16.2 mM. Determined by lH NMR analysis of the reaction mixture. CEstimated by GPC based on polystyrene standards.

c-1

I

I

I

28

3o

32

5.9

2.2

0.9

I

Elution Volume in m l

M

~ l o 4~

~

x

Figure 2. Two-stage polymerization of methyl methacrylate (MMA)in CH2Cll at room temperature withthe enolatduminum porphyrin (2, R = Me)-methylaluminum bis(2,4,6-tri-tertbutylphenolate) (30 (1.011.0) system. GPC profiles of the polymers formed at the first stage (I) ([MMAldfl (X = Me)lo = 50,100% conversion,Mn= 7000, Mw/Mn= 1.12) and the second stage (11) ([MMA]d[Z]o = 200,100% conversion, M , = 47 600, MwIM,, = 1.05).

phenolate unit of 3b into the polymer terminal. Furthermore, with 3b alone, the polymerization of MMA, did not take place at all under identical conditions. In addition to MMA, a variety of methacrylic esters could be polymerized rapidly to the corresponding polymers with narrow MWDs in the presence of methylaluminum bis(2-tert-butyl-4-methoxyphenolate) (3c)(1equiv relative to 2). The successful examples include ethyl methacrylate (EMA), isopropyl methacrylate ('PMA), n-butyl methacrylate ("BMA), isobutyl methacrylate ('BMA),benzyl methacrylate (BnMA), and dodecyl methacrylate (CI~MA),where the M, values were all close to the predicted values (M,(calcd)) with the M,IM, ratios below 1.1 (runs 1-4, 6,and 7,Table 111). The polymerization of tert-butyl methacrylate (tBMA) (run 5) is an only exceptional case, where the monomer conversion hardly increased even after 24 h. The polymerization in the presence of bulky methylaluminum diphenolates proceeds with living character. Clear evidence for this was given by the sequential twostage polymerization of MMA with the 2 (R= Me)-Sf (1 equiv/l equiv) system in CH2C12 at room temperature (Figure 2),where 50 equiv of MMA with respect to 1 (X = Me) were polymerized up to 100% conversion (90 a) at

1

:

.---

100

.-

1

C

E>

80

'

0

v

60

40

.

d

1

0 0

120

130

140

150

Reaction time in min

Figure 3. Polymerizations of tert-butyl methacrylate ('BMA) (A) and methyl methacrylate (MMA) (B)initiated with (TPP)AlMe (1, X = Me; [monomerld[l (X = Me110 = 100, 11 (X = Me110 = 17.8 (A) and 19.6 (B)mM, CHzCl2 as solvent, room temperature). Effect of trimethylaluminum (Me& ([Medld [2]0 = 3.0) on the rate of polymerization.

the first stage, and after a 4-h interval at 25 OC, 200 equiv of MMA were charged to the mixture. In spite of a long interval between the first and second monomer feeds, the second-stage polymerization ensued with heat evolution, and was completed within 608. The GPC profiie showed a clear increase in molecular weight of the polymer from 7000 (the first stage, Figure 2 (I)) to 47 600 (the second stage (II)),retaining the narrow MWD (M,/M,,from 1.12 to 1.05). Polymerization of Methacrylic Esters via Enolatoaluminum Porphyrins (2) in the Presence of Trialkyl- and Triarylaluminum Compounds. Similarly to methylaluminum diphenolates (3), trialkylaluminums such as trimethylaluminum (Me&), triisobutylaluminum ( ' B u d ) , and triphenylaluminum ( P W ) , by themselves, have no ability to initiate the polymerization of methacrylic esters under ordinary conditions. For investigating the acceleration effects of trialkylaluminums, the polymerization of tert-butyl methacrylate (tBMA)was first chosen, since it proceeds slowly even in the presence of methylaluminum diphenolates (3)as Lewis acids. However, quite unexpectedly, the polymerization was dramatically accelerated when M e d l was added to the system. As exemplified in Figure 3A, the polymerization of tBMA (100equiv) with 1 (X = Me), which had proceeded to 7% conversion by 2-h irradiation at 35 OC, proceeded rapidly with vigorous heat evolution as soon as 3 equiv of M e d l with respect to2 (R = tBu)was added at room temperature, and the complete consumption of tBMA was attained within only 5 min. The produced polymer exhibited a unimodal, sharp GPC peak, and the Mn value increased linearly along the theoretical line (dotted, Figure 4) calculated from the monomer-to-initiator (1,X = Me) mole ratio, keeping the M,/M, ratio in the range 1.05-1.06. It was also noted here that the addition of Me& is accompanied by a rapid color change of the system from dark red-purple due to 2 to blue-purple characteristic of methylaluminum tetraphenylporphyrin (1, X = Me). In contrast, when the monomer was methyl methacrylate (MMA),the polymerizationwas terminated upon addition of Me& under similar conditions. For example, when 3 equiv of Me&l with respect to 2 (R = Me) was added to thesystem([MMA]d[l (X= Me)]o= 100,995 conversion), heat evolution, although observed at the very early stage, subsided within only 1-2 min. At this stage, the color of the solution had already turned blue-purple. The monomer conversion after 5 min was only 30%, and it no longer increased upon prolonged reaction for 2 h (Figure 3B).

T

A

Macromolecules, Vol. 26, No.13, 1993

k '*' d

20

(b)

3*0

theoretical

0


40

60

,,,'

80

100

Conversion in

YO

Figure 4. Polymerization of tert-butyl methacrylate (tBMA) initiated with (TPP)AlMe (1, X = Me) via an enolatoaluminum porphyrin (2, R = 'Bu) in the presence of trimethylaluminum (Me&, ['BMAld[l (X = Me)ld[Medllo = 100/1.0/3.0,[110= 17.8mM, CH2Cl2 as solvent, room temperature). Relationship between M. ( 0 )(Mw/Mn( 0 ) )of the polymer and conversion.

~~

Table IV. Polymerization of Methyl Methacrylate (MMA) via an Enolatoaluminum Porphyrin (2, R = Me) in the Prerence of Triallrvlaluminums ( R A P run R&l timed/min convbl% Mnc MW/Mnc ~

1

Me&l

2

'BW

3

Ph.4

0 10 30 0 10 30 0 30 60 90

4 9 9 6 63 64 3 46 79 100

19500 20200

1.37 1.41

8600 16700 22300

1.19 1.17 1.18

O In CHzCla under nitrogen, [MMAIJtl (X = Me)ld[R&llo= 200/1.0/3.0, [l (X = Me110 = 16.2 mM. Determined by lH NMR analysis of the reaction mixture. cEstimated by GPC based on polystyrene standards. After addition of R&l.

3o

32

34

Elution Volume

5.9 2.2 0.9

0.4

M~~~ x 1 0 4

28

in mL

Figure 5. Polymerizations of tert-butyl methacrylate ('BMA) initiated with (TPP)AlMe (1, X = Me; ['BMAId[l (X = Me)],, = 100, [llo = 17.8 mM, CHzCll as solvent). GPC profiles of the polymers formed (I) after 90-h irradiation at 35 "C,25% conversion,and (11) 60min after addition of trimethylaluminum ( M e a ; [Meald[2 (R = 'Bu)]o = 0.25) at room temperature,

100% conversion.

*

Use of bulkier ' B u d in place of M e d l for the polymerization of MMA ([MMAId[2 (R = Me)ld[iBu&l~ = 200/1.0/3.0)gave an essentially similar result, although the monomer conversion finally attained was higher (647%) (run2,Table IV). The M , of the polymer formed at this conversion (20200)was much higher than that expected from the ratio of MMA reacted to 1 (X= Me) (12 800), and the MWD was broad (Mw/Mn= 1.41). On thecontrary, when Ph& (3 equiv with respect to 2 (R = Me)) was added at room temperature to the system ([MMAld[ll~ = 200,3% conversion), a fairly rapid polymerization took place with heat evolution, and the monomer was completely consumed within 90min (run 3,Table IV). The Mn of the produced polymer (22300)was close to the expected value of 20 OOO, and the MWD was satisfactorily narrow (Mw/ Mn = 1.18). In this case, the system retained the original color characteristic of 2 (R = Me) throughout the polymerization. At a lower temperature, in contrast, even Me& was able to accelerate the polymerization of MMA. At -40 OC, for example, the polymerization with 1(X= Me) alone as initiator ([MMAlo/[l (X = Melo = 100) in CHzClz proceeded to only 11 % conversion in 1 h, but after 3 equiv of Me& was added to the system, it attained 91% conversion in the next 1 h. Furthermore, the polymer formed here had a narrow MWD as indicated by the Mw/ Mn ratio of 1.17,and the& value (9700)was close to that expected (9100)from the mole ratio of the monomer r e a d to the initiator. In this case, the polymerization

0,'

was observed to proceed without any change in color of the system. Thus, the polymerization temperature is one of the important factors to achieve the clean and rapid polymerization with trialkylaluminums. In this regard, when 0.25 equiv of Me& with respect to the growing species (2) was added at room temperature to the polymerization system of 'BMA ([tBMAld[l (X= Me110 = 100)a t 25% conversion, a part of the prepolymer (peak a in Figure 5 (I))remained unreacted even after the complete monomer consumption was attained (Figure 5 (11)). When the amount of Me& relative to 2 was increased from 0.25to 0.5,peak a became much smaller, while the molecular weight corresponding to the major peak (peak b) decreased. On the other hand, when the ratio [ M e a l d [%loexceeded unity (1.5and 3.0),peak a disappeared completely to give aunimodal MWD product. At [Me&ld[tI~ 2 1,the Mn value of the final product was no longer affected by the ratio [Me&I0/[210 (Figure 6). Thus, the number of polymer molecules versus 2 (NPINwp),calculated from



Scheme VI1

0.0 I 0.0

'

1 .o

0.0

3.0

2.0

6

[Me3All,/[2 (R = Me)lo

Figure 7. Polymerization of methyl methacrylate (MMA) initiated with (TPP)AIMe(1,X = Me) via an enolatoaluminum porphyrin (2, R = Me) in the presence of trimethylaluminum ( M e a ; [MMA]$[l (X = Me110 = 100, [ l l o = 19.6 mM, CHzClz aa solvent,-40 "C).Correlations of the number-averagemolecular weight (Mdand the ratio of the numbers of polymer molecules to 2 (Np/N~p) with the initial mole ratio of M e a to 2. (A)

c e C=$

b

c=o c-0

*

--r

200

7

160

--?-

120

--_ 80

-40

-0 PPm

Figure 8. 13C NMR spectra in CDzClz at 25 "C of (A) methyl methacrylate (MMA) and (B)an equimolar mixture of MMA and methylaluminum bis(2,4,6-tert-butylphenolate)(30. the Mn of the polymer corresponding to peak b (Figure 5 (11)) and the initial monomer-to-initiator mole ratio, increased with increasing the ratio [Me&10/[210 up to unity, but no further increase in N p / N ~ p presulted after the ratio [Me+41]0/[210exceeded unity. In sharp contrast, in the case of the polymerization of MMA at -40 "Cwith varying mole ratios of [Me&l0/[210, only the rate of polymerization was changed, but in any case the produced polymer exhibited a unimodal, sharp GPC peak with the Mnvalue close to that expected from the formation of one polymer molecule from every molecule of 2 (R = Me) (Figure 7). NMR Studies. (I) Interaction between Monomer and Organoaluminum Compounds. In the 'H NMR spectrum of an equimolar mixture of MMA and 3f in CD2Clz at 25 "C, all the signals due to MMA clearly shifted downfield from those of MMA alone, where the signals due to CH2 (6 6.00 and 5.491, CH30 (6 3.66), and CH3 (6 1.86) of MMA appeared at 6 6.41 and 5.81,3.92, and 1.98, respectively. In the 13C NMR spectrum, the signals due t o , e . g . , C=O (a,6 168.31, CH2 (c, 6 125.61, and CH30 (d, 52.2) of MMA also shifted downfield to 6 173.3 (a'), 133.8 (c'), and 56.5 (d), respectively, in the presence of an equimolar amount of 3f (Figure 8B). As for the signals of 3f,14 the C-0 signal of the phenolate ligands most clearly shifted from 6 152.6 to 6 155.7 (A). These

observations demonstrate the occurrence of a coordinative interaction between the carbonyl group of MMA and 3f. In the 13C NMR spectrum of an equimolar mixture of MMA and Me& in CDzClz at 25 OC, a downfield shift was again observed for the C E O signal of MMA from 6 168.3 to 6 172.2. On the other hand, for an equimolar mixture of MMA and P h d , the downfield shift of the MMA C 4 signal was very small (0.1 ppm) compared with those for the MMA-3f and MMA-MesA1 systems under similar conditions. (11) Ligand Exchange Reaction between 2 and Organoaluminum Compounds. A CsD6 solution of an equimolar mixture of the living polymer of MMA (2, R = Me; [MMAld[l (X= Me110 = 10,100% conversion) and 3f was studied by lH NMR. If the ligand exchangereaction took place between 2 (R = Me) and 3f, 1 (X= Me) and/or 6 (R1 = R2 = R3 = tBu) should be formed (Scheme VII), which are easily detectable by the characteristic signals at 6 -5.79 and -0.26 due respectively to the MeAl group of 1(X= Me) and the tBu group of 6 (R1= R2 = R3 = tBu). However, throughout the observation over a period of 24 h at 25 O C , neither of these two signals was detected. In sharp contrast, when Me& was mixed with 2 (R = tBu; [tBMAld[l (X= Me110= 5,100% conversion)under similar conditions, the color of the solution immediately turned from dark red-purple to blue-purple. The lHNMR spectrum of this mixture in c& showed the appearance of a new signal due to 1 (X = Me) (f, 6 -5.79) with the intensity ratio of 3:8 to that of the pyrrole 6protons (8H) of the porphyrin ligand, while a characteristic signal at 6 -0.33 ppm due to the enolate 'Bu group of 2 (R = tBu) (a)15 completely disappeared (Figure 9). This observation indicates the occurrence of a rapid ligand exchange between2 (R= tBu) and Me& (SchemeVIII). The ligand exchange reaction also occurred in the 2 (R= Me)-Me& system. Figure 10A shows the lH NMR spectrum in CDICl2 at 25 "C of 2 (PMMA-ds),prepared from MMA-da and 1 (X = Me) with the ratio of 101, where only the terminal methyl group originating from 1 (X = CH3) (6 0.70) is observed besides the signals due to the porphyrin ligand of the aluminum enolate species (a, 6 9.10 (pyrrole 8-23); b, 6 8.21 (Pho o-H); c, 6 7.80 Ph m,p-H)). Upon addition of 3 equiv of Me& at 25 "C to this enolate solution, the spectrum was changed to Figure 10B, where a new signal (f') due to the MeAl group of 1 (X = Me) appeared in addition to some new signals (e', g') assignable to the MeAl groups of the dimethylaluminum enolate and excess Me3Al. Moreover, the signals due to the porphyrin ligand shifted slightly but definitely to those for 1 (X= Me) (a', 6 9.05 (pyrrole 6-H); b', 6 8.19 (Ph o-H); c', 6 7.77 (Ph m,p-H)),although the signal due to the terminal methyl group shifted only little from d to d', as expected. Contrary to the above results at 25 "C, the ligand exchange reaction (Scheme VIII) was found to be greatly

Macromolecules, Vol. 26, No. 13,1993


b

a

f

p'

d2,

(R = 'Bul

1 (X

I

Me)

z f

I

I

I

I

T

,

,

I

,

,

0.0

1 .Q

I

I

I

I

-1.0

I

___

-

a

-

m

A 1

I

4

4

!l/-----r -2.0

-6.0

Sin ppm

Figure 9. 1H NMR spectra in CD2Clz at 25 "C of (A) the enolatoaluminumporphyrin (2, R = 'Bu; ['BMAld[l (X= Me110 = 5,100% conversion) and (B) the reaction mixture of 2 (R = #Bu)and trimethylaluminum ( M e a ; [Me&ld[210 = 3.0, 1230 = 25.0 mM).

*Q

(A)

I'

a,

10

8

6

4

2

0

.2

-4

.6

-8 ppm

Figure 10. lH NMR epectra in CDzCl2 at 25 O C of (A) the enolatoaluminumporphyrin (2, PMMA-de; [MMA-dald[l (X= Me)lo = 10,100%conversion) and (B)the reaction mixture of 2 (PMMA-dd and trimethylaluminum ( M e a ; [Me&ld[2Io = 3.0, [2]0 = 25.0 mM).

Scheme VI11

retarded by lowering the temperature. Figure 11 shows the change in mole fraction of 1 (X = Me): [l (X= Me)]/ ([l (X = Me)] + [2 (PMMA-ds)]),as determined from the relative intensity of the signals f' to a and a' (Figure lo), after addition of 3 equiv of M e d l to 2 (PMMA-ds)at -40 "C. Although a trace amount of 1 (X = Me) was accidentally generated at the initial stage until the probe

temperature was settled, its mole fraction was never increased throughout the observation over a period of 75 min. On the other hand, when the probe temperature jumped at 25 O C , a steep increase in the mole fraction of 1 (X = Me) was observed. Polymerizations of Methacrylic Esters Initiated with 1- tert-Butoxy-1-[(diethylaluminio)oxy]-2-methylpropene (5). In connectionwith the ligand exchange reaction between the enolatoaluminumporphyrin (2) with MegAl (Scheme VIII), the diethylaluminum enolate (5), a model compound of the exchanged product, was synthesized according to Scheme VI, and tested as initiator for the polymerizations of methyl methacrylate (MMA) and tert-butyl methacrylate (tBMA). For example,when 100 equiv of MMA was added to a CHzClz solution of 5 at room temperature, the polymerization was actually initiated with heat evolution, but terminated only at 10.37% conversion, affording a low molecular weight PMMA possessing a very broad MWD as indicated by the Mw/Mu ratio of 5.6. On the other hand, as for tBMA under similar conditions ([tBMAld[510= 1001, the enolate (5) brought about a very rapid polymerization, and the complete monomer consumption was attained within minutes, affording the narrow MWD polymer (Mw/Mu= 1.05) with an Muof 34 600. The observed Muvalue, although higher than expected, was observed to increase almost linearly to 77 100 and 102 000,retaining the narrow MWD (Mw/ Mn= 1.05-1.13),'6 when the initial monomer-to-initiator mole ratio was increased to 200 and 300, respectively. Mechanistic Aspects. To summarize the above results (Tables I, 11,and IV), methylaluminum ortho-substituted phenolates (3a-3g), methylaluminum bis(tripheny1methanolate) (4b), and PhgAl as Lewis acid additives can dramatically acclerate, even at room temperature, the polymerization of MMA via 2 without damaging the living character of polymerization. On the other hand, when methylaluminumphenolates bearing no ortho substituents (3h,3i),methylaluminum bis(diphenylmethano1ate)(4a), Me& and 'BusAl are used under similar conditions, the polymerization is terminated. When the temperature is lowered to, e.g., -40 "C, even M e a is usable for accelerating the polymerization of MMA without any loss of the living character. Together with the results of the NMFt



Conclusion The aluminum porphyrin initiators (1) provide one of the bulkiest families of nucleophilicgrowingspecies,which are very reluctant to undergo degradative reaction with bulky Lewis acids even at room temperature. By taking the advantage of this, the Lewis acid-assisted high-speed living anionic polymerization of methacrylic esters was realized (Figure 12). By lowering the polymerization temperature, even Lewis acids with poor steric bulk such as trialkylaluminums were usable as the accelerators.

Accelerated by Monomer Activation .-__

$

Bulky Nucleophile

Side Reaction

Bulky Lewis Acid

l? Suppressed by Steric Repulsion

I

Figure 12. Basic concept of the Lewie acid-assisted 'high-speed living anionic polymerization" of methacrylic esters. investigations, the accelerated living polymerization of MMA with the %-Lewis acid (3a-3g) systems is concluded to be the result of (1)coordinative activation of MMA for the nucleophilicattack of 2 (Figure 12)and (2)suppression of the undesired degradative attack of 2 to the Lewis acidic center (Scheme VII) because of the steric repulsion between the bulky porphyrin ligand of 2 and the substituents at the ortho positions of the phenolate ligands of 3a-3%. The same may be true for the 2-Ph3Al system. When the substituents of the Lewis acid are smaller as typically in the case of Me&, the undesired ligand exchange reaction (Scheme VIII) competitively occurs, leading to the formation of dimethylaluminum enolate (7) at the expense of the change of 2 into methylaluminum porphyrin (1, X = Me) (Figure 10). As already reported, 1 (X = Me) has no ability to initiate the polymerization of methacrylic esters without visible light irradiation. Furthermore, dimethylaluminum enolate is not a suitable growing species for the living polymerization of MMA, taking into account the fact that the polymerization initiated with the diethylaluminum enolate (5) at room temperature is terminated at the very early stage. Therefore, the ligand exchange reaction (Scheme VIII) leads to the termination of chain growth in the polymerization of MMA. Similarlyto the above, the ligand exchangereaction (Scheme VIII) actually occurs at room temperature when the monomer is tert-butyl methacrylate VBMA) (Figure B), but the resulting dimethylaluminum enolate (7, R = tBu) can initiate the rapid polymerization of tBMA in a fairly controlled fashion, considering the quantitative formation of a narrow MWD P'BMA using 5 as the initiator. Therefore, when Me& is added to the living PtBMA (2, R = tBu) at the ratio below unity, only part of the polymer molecules are transformed into the more active form 7 (R = tBu), and the formation of a bimodal MWD PtBMA results (Figure 5). This is also supported by the results shown in Figure 6. At a lower temperature such as -40 "C, the ligand exchange reaction (Scheme VIII) is very much reluctant to occur (Figure ll),so that the polymerization is accelerated by M e d l without losing the living character (Figure 7), similarly to the case using the 2-bulky 3 systems.

Acknowledgment. The present work was partly supported by Grant-in-Aid No. 04204006 for Scientific Research on Priority Area, the Ministry of Education, Science and Culture, Japan. T.A. thanks the Iketani Science and Technology Foundation and IC1 Scientific Research Fund for financial support. The authors also thank Professor Koichi Hatada and Dr. Tatsuki Kitayama of Osaka University for their generous suggestion on the preparation of methyl methacrylatede, and Messrs. Munetoshi Morita and Yasunori Hosokawa for their active collaboration. References and Notes (1) For example, see: (a) Suzuki, M.; Yanagisawa,A.; Noyori, R.

TetrahedronLett. 1982,23,3595.(b)Volkmann, R.A.; Davis, J. T.; Meltz, C. N. J. Am. Chem. SOC.1983,105,6946.(c) Em, M.J.; Wrobel, J. E.; Ganem, B.J. Am. Chem. SOC.1984,106, 3693. (d) Corey, E.J.; BO=, N.W . TetrahedronLett. 1986,26, 6015,6019. (e) Maruoka, K.; Ito, T.; Sakurai, M.; Nonoshita, K.; Yamamoto, H. J. Am. Chem. SOC.1988,110,3588. (2) . . Kitavama. T.: Shinozaki. T.: Masuda,. E.:. Yamamoto.. M.:. HaGda, K.Polym. Bull. 1988; 20,505. (3) Ballard. D. G. H.: Bowles. R. J.: Haddleton, D. M.: Richards, S. N.; Sellens, R.;.%ose, D. L.Macromolecules 1992,26,6907. (4) Kuroki, M.; Watanabe, T.; Aida, T.; Inoue, S. J. Am. Chem. SOC.1991,113,5903. (5) Kuroki, M.;Aida, T.; Inoue, S. J. Am. Chem. SOC.1987,109, 4737. (6) Adachi, T.; Sugimoto, H.; Aida, T.; Inoue, S. Macromolecules 1992,25,2880. (7) Shimasaki, K.;Aida, T.; Inoue, S. Macromolecules 1987,20, 3076. (8) Adler, A. D.; Longo,F. R.;Finarelli,J. D.; Goldmacher,J.;Assour, J.; Korsakoff, L.J. Org.Chem. 1967,32,476. (9) Hatada, K.; Kitayama, T.; Fujikawa, K.; Ohta, K.; Yuki, H. ACS Symp. Ser. 1981,166,327. (10)Wittig, G.; Wittenberg, D. Ann. Chem. 1967,606,1. (11) Hirai, Y.; Murayama, H.;Aida,T.; Inoue, S. J. Am. Chem. SOC. 1988,110,7287. (12) (a)Strowieyski,K. B.;Pasynkiewicz,S.; Skowronska-Ptasinska, M. J. Organomet. Chem. 1976,90, C43. (b) See also ref le. (13) (a) Danishefeky, S.;Kitahara, T.; Tsai, M.; Dynak,J. J. Org. Chem. 1976,41, 1669. (b) Sturm, T.-J.; Marolewski, A. E.; Rezenka, D. S.; Taylor, S. K. J. Org. Chem. 1989,54,2039. (14) The assigment was made according to Maruoka,K.;Araki, Y.; Yamamoto, H. J. Am. Chem. SOC.1988,110,2650(eupplementary material). (15) 1H NMR for 2 (R = 'Bu) (C&): S 9.36 (pyrrole 8-H), 8.48 (Ph o-H),7.76(Phm,p-H),2.7-2.3(mainchainCH2),1.9-1.4(main chain CHs and C(CH&), 0.96 (terminal CHs; e in Figure 9), 0.72 and 0.70 ( 4 C H s ; b), 0.51 (-C(CHs)CH&CHs; d), 0.430.40( 4 C H 2 ;c), 0.0to -0.2 (=C(CHS)CH,CCHS; d), -0.28 and -0.33 (=COC(CHs)s; a), -1.00 and -1.09 (==CCHs; b), -1.14 (=CCH2; c). (16) A smallshoulder was observed on the higher molecular weight side of the main peak in GPC.

High-speed Living Anionic Polymerization of Methacrylic Esters with

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